317 From past to present: biodiversity in a changing delta K. Troost, M. Tangelder, D. van den Ende & T.J.W. Ysebaert werkdocumenten Wettelijke Onderzoekstaken Natuur & Milieu WOt From past to present: biodiversity in a changing delta The ‘Working Documents’ series presents interim results of research commissioned by the Statutory Research Tasks Unit for Nature & the Environment (WOT Natuur & Milieu) from various external agencies. The series is intended as an internal channel of communication and is not being distributed outside the WOT Unit. The content of this document is mainly intended as a reference for other researchers engaged in projects commissioned by the Unit. As soon as final research results become available, these are published through other channels. The present series includes documents reporting research findings as well as documents relating to research management issues. This document was produced in accordance with the Quality Manual of the Statutory Research Tasks Unit for Nature & the Environment (WOT Natuur & Milieu). WOt Working Document 317 presents the findings of a research project commissioned by the Netherlands Environmental Assessment Agency (PBL) and funded by the Dutch Ministry of Economic Affairs (EZ). This document contributes to the body of knowledge which will be incorporated in more policy-oriented publications such as the National Nature Outlook and Environmental Balance reports, and thematic assessments. From past to present: biodiversity in a changing delta K. Troost M. Tangelder D. van den Ende T.J.W. Ysebaert Werkdocument 317 Wettelijke Onderzoekstaken Natuur & Milieu Wageningen, December 2012 Abstract Troost, K., M. Tangelder, D. van den Ende & T.J.W. Ysebaert (2012). From past to present: biodiversity in a changing delta. Wageningen, Statutory Research Tasks Unit for Nature & the Environment (WOT Natuur & Milieu), WOt-werkdocument 317. 75 p. 38 Figs; 1 Table; 53 Refs; 2 Annexes. A large-scale coastal engineering project (the ‘Delta works’) changed large-scale, dynamic estuarine nature in the southwest of the Netherlands into a diverse mosaic of ecosystems with different characteristics. This led to a suite of ecological problems, which is why plans are made to restore estuarine dynamics. Until today the effect of the Delta works on biodiversity in the subsystems is still poorly understood. We combined long-term datasets on macrobenthos, fish, birds and key species and present reliable and factual information on changes in biodiversity in the Southwest Delta in the past decennia in relation to the Delta works and other developments. Effects of the Delta works on biodiversity are highly diverse and depend on many different factors and histories specific for the different water bodies. If connections are restored, effects on species richness and biodiversity will depend on the specific characteristics of the separate basins. Because restoration of estuarine dynamics likely occurs on a reduced scale, effects on biodiversity may only be modest. However, effects on the occurrence of rare species of the brackish and intertidal transition zones may be more significant. It is recommended to study this further. Key words: biodiversity, restoration of estuarine dynamics, Delta works, long-term trends, species richness Trefwoorden: biodiversiteit, zuidwestelijke Delta, Deltawerken, herstel estuariene dynamiek, lange termijn trends, soortenrijkdom © 2012 IMARES Wageningen UR Postbus 77, 4400 AB Yerseke Phone: +31 (0)317 48 09 00 E-Mail: [email protected] www.imares.wur.nl The Working Documents series is published by the Statutory Research Tasks Unit for Nature & the Environment (WOT Natuur & Milieu), part of Wageningen UR. This document is available from the secretary’s office, and can be downloaded from www.wageningenur.nl/wotnatuurenmilieu Statutory Research Tasks Unit for Nature & the Environment, P.O. Box 47, NL-6700 AA Wageningen, The Netherlands Phone: +31 317 48 54 71; Fax: +31 317 41 90 00; e-mail: [email protected]; Internet: www.wotnatuurenmilieu.wur.nl All rights reserved. No part of this publication may be reproduced and/or republished by printing, photocopying, microfilm or any other means without the publisher’s prior permission in writing. The publisher accepts no responsibility for any damage ensuing from the use of the results of this study or from the implementation of the recommendations contained in this report. F-0008 vs. 1.9 [2012] Project WOT-04-011 – 036.06 [Werkdocument 317 ­ December 2012] 4 WOt-werkdocument 317 Contents Summary 7 Samenvatting 9 1 Introduction 11 2 Materials and Methods 13 2.1 Data collection and availability 13 2.2 Functional groups 14 2.3 Data processing, statistics and presentation 15 3 Overview of water bodies and major changes 17 3.1 Oosterschelde 17 3.2 Grevelingen 20 3.3 Lake Veere 22 3.4 Haringvliet 23 4 Analysis results 25 4.1 Oosterschelde 25 4.2 Grevelingen 29 4.3 Lake Veere 34 4.4 Haringvliet 37 4.5 Seagrass meadows, sea mammals and salt marshes 39 4.6 Comparisons between water bodies 41 5 Discussion and conclusions 45 5.1 Oosterschelde 45 5.2 Grevelingen 46 5.3 Lake Veere 47 5.4 Haringvliet 48 5.5 Conclusions 48 5.6 Recommendations for further research 50 Acknowledgements 52 References 53 Appendix 1 List of species and their feeding guilds 57 Appendix 2 Significance tables (TrendSpotter) 69 Summary A large-scale coastal engineering project (the ‘Delta works’) drastically changed the appearance, hydromorphology and ecology of the Rhine-Meuse and Scheldt delta in the southwest of the Netherlands. A formerly estuarine delta with multiple branches was fragmented by dams into several smaller areas of which many lost their estuarine character. Large-scale, dynamic estuarine nature changed into a diverse mosaic of ecosystems with different characteristics. However, in recent years it became clear that there is also a downside to the Delta works. Reduced dynamics resulted in several ecological problems, such as erosion of tidal flats in the Oosterschelde estuary, blooms of cyanobacteria in Lake Krammer-Volkerak, and oxygen deficiency in Lake Grevelingen. To address these problems, as well as future effects of climate change and sea level rise, possibilities for restoring estuarine dynamics, salinity gradients and connectivity between water bodies are currently investigated (Deltaprogramma, Stuurgroep Zuidwestelijke Delta). To be able to predict consequences for species biodiversity we need to know todays biodiversity, as well as how biodiversity of the different water bodies, and the delta area as a whole, changed due to the long-term effects of the ‘Delta project’. This study addresses the question what will be gained and what will be lost if we restore estuarine dynamics. This question is answered in two reports, financed by the Ministry of Economic Affairs (EZ). The first of the two reports shows how the biodiversity of some main species groups (birds and fish) in each subsystem evolved into today’s state, and how this relates to the biodiversity of the entire Delta area as a whole. Based on these results it is questioned whether restoration of estuarine dynamics will lead to an increase or decrease in biodiversity, species richness, and overall robustness (Tangelder et al., 2012). The second report is the one presented here. The study was commissioned by the Netherlands Environmental Assessment Agency (Planbureau voor de Leefomgeving, PBL) with the goal to make a more detailed comparison possible between on the one hand an open delta with connections between the different water bodies and with the river systems and North Sea, and on the other hand a fragmented delta as was created by the Delta works. Water bodies studied in detail are the Oosterschelde estuary and the Lakes Grevelingen, Veere and Haringvliet. We combined available long-term datasets on macrobenthos, fish, birds, and key species (sea grass and sea mammals) with the aim to present reliable and factual information on changes in biodiversity in the Southwestern Delta in the past few decennia, and how the Delta works influenced it. In many cases there were no time series available that cover the period around, or just after, the construction of the Delta works. Before describing analysis results we therefore first give a literature overview of documented changes as a consequence of closing the Oosterschelde from riverine input, completion of the Oosterschelde storm surge barrier, and creation of the Lakes Grevelingen, Veere and Haringvliet. The results clearly illustrate the main problems of the Delta works: almost no migration possibilities for fish, a loss in estuarine dynamics causing a reduction in pioneer vegetations and breeding habitats on bare grounds as well as bad water quality, and a loss in species strictly associated with the intertidal and brackish zones in estuarine salinity gradients. If connections are to be restored (even to some extent) between saltwater and freshwater systems, allowing for migration and salinity gradients as well as some tidal movement, this is likely to lead to a higher species richness locally. Overall biodiversity in the different water bodies and the Southwestern Delta as a whole may only change slightly. However, restoration of connections and estuarine gradients and dynamics is likely to occur on a scale that is much more reduced in comparison with the situation before the Delta works. The positive effects on biodiversity caused by these measures may therefore be modest. On the other hand, effects on the occurrence of species and communities exclusively occurring in the intertidal and brackish transition zones that have become more rare due to the Delta works, may be more significant. It is recommended to study this effect in more detail in a follow-up study. From past to present: biodiversity in a changing delta 7 8 WOt-werkdocument 317 Samenvatting De zuidwestelijke Delta is als gevolg van de Deltawerken drastisch veranderd. De voorheen grootschalige estuariene natuur werd omgevormd in een gevarieerd mozaïek van verschillende, van elkaar gescheiden, waterbekkens met grote verschillen in abiotische en ecologische karakteristieken.